Problem: The scale on a map is 3cm : 8km. If the distance between two cities is 40km, how far apart in cm are the two cities on the map?
The scale means that every 3cm on the map corresponds to an actual distance of 8km. An actual distance of 40km is the same as 5 $\cdot$ 8km. The distance between the two cities on the map is 5 $\cdot$ 3cm, or 15cm.